Protection of oxide superconductors from degradation

ABSTRACT

A method of treating an oxide superconductor to prevent degradation thereof comprises treating the superconductor with a solution of a metal which is capable of being oxidized by the superconductor to form on those regions of the surface area of the superconductor treated with the solution an insulating, non-porous protective film of an oxide which is stable against further oxidation.

The present invention relates to the protection of oxide superconductorsfrom degradation.

BACKGROUND OF THE INVENTION

The recently discovered oxide superconducting ceramic materials, withsuperconducting transition temperatures (T_(c)) in the range above thatof liquid nitrogen (77° K), are technologically very exciting materials.However, they are not known to suffer extensive chemical degradation inthe atmosphere, particularly in the presence of water vapour and carbondioxide. In addition, they are highly oxidising and are able to oxidisemetals, including gold, in bulk solution and in condensed moisturefilms. This reactivity will limit their eventual use and methods ofeliminating the degradation must be sought prior to commercialexploitation.

It is therefore an object of the present invention to provide a methodof protecting such superconductors from degradation.

SUMMARY OF THE INVENTION

According to the present invention there is provided a method oftreating an oxide superconductor to prevent degradation thereof themethod comprising treating the superconductor with a solution of a metalwhich is capable of being oxidised by the superconductor to form onthose regions of the surface area of the superconductor treated with thesolution an insulating, non-porous protective film of an oxide which isstable against further oxidation.

The invention also provides an oxide superconductor in which at least aportion of the surface area of the superconductor is coated with aninsulating, non-porous protective film of a metal oxide which is stableagainst further oxidation by the superconductor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention is applicable particularly to the treatment ofcopper-containing superconducting oxides. All oxide superconductorscontaining copper and/or an oxidised cuprate peroxide complex will besubject to degradation as described above and will therefore be suitablefor treatment. Classes of copper containing superconductors to which theinvention is applicable are given in Table I, together with specificexamples of each class.

                  TABLE 1                                                         ______________________________________                                        Classes of super conductor                                                                      Examples                                                    ______________________________________                                        La--Ba--Cu--O     La.sub.1.8 Sr.sub.0.2 CuO.sub.4 - X                         Y--Ba--U--O       Y Ba.sub.2 Cu.sub.3 O.sub.7 - X                             (Pb) Bi--Sr--Ca--Cu--O                                                                          Bi.sub.2 Sr.sub.3 - X Ca.sub.x Cu.sub.2 O.sub.8 - X         Tl--Ba--Ca--Cu--O Tl.sub.2 Ba.sub.2 CaCu.sub.3 O.sub.10                       ______________________________________                                                          - X                                                     

In addition rare earth, and other, elements may dope the above.

It has been found that the provision of the metal oxide film acts as aneffective barrier to protect the superconductor from corrosion andenvironmental degradation. The oxide film is so effective that untreatedsuperconductors which would normally be attacked and quickly destroyedby atmospheric moisture are (when treated in accordance with theinvention) totally stable when exposed to the atmosphere.

The method of the invention can most conveniently be effected by dippingthe superconductor into the solution of the metal ions which give riseto the protective oxide film and simultaneously applying an appropriateoxidising electrochemical potential. For superconductors which have adegree of porosity, the solution will penetrate the interstices withinthe superconductor so that the protective metal oxide is deposited onthe interior surface area.

The metal ion in the treatment solution must be one which can beoxidised on the superconductor surface to give an oxide film which isstable against oxidation by the highly oxidising superconductor.Suitable metal ions include Mn²⁺, Ti³⁺, Pb²⁺ and Sn²⁺.

In the case where the metal ion in an aqueous treatment solution is Mn²⁺and the superconductor contains Cu³⁺ ions, the reaction process isessentially

    Mn.sup.2+ +2H.sub.2 O=MnO.sub.2 +4H.sup.+ +2e.sup.-

In the above example, the oxidation of Mn²⁺ to MnO₂ may also be effectedby an appropriate applied external potential (voltage).

The preferred concentration of the metal ion will depend on theparticular metal, and the preferred ranges for Mn²⁺, Ti³⁺, Pb²⁺ and Sn²are given in Table 2 below which also includes the preferred (acidic) pHof the medium.

                  TABLE 2                                                         ______________________________________                                        Ion           Concentration                                                                             pH                                                  ______________________________________                                        Mn.sup.2+     0.01 to 0.5M                                                                              <5                                                  Ti.sup.3+     10.sup.-3 to 0.1M                                                                         <2                                                  Pb.sup.2+      0.01 to 0.05M                                                                            <6                                                  Sn.sup.2+     10.sup.-3 to 0.01M                                                                        <1                                                  ______________________________________                                    

In all cases the metal ion may be used in aqueous solution or a lowwater activity solvent (e.g. acetic acid). Preferably the solution is abuffered solution, and preferably a potential of >0.8 V (referred to theStandard Hydrogen Scale) is applied.

Preferred treatment solutions are equimolar Mn acetate and acetic acid(pH ca 4 to 5), equimolar Pb acetate and acetic acid (pH ca 4 to 5), orsolutions of SnCl₂ or TiCl₃ within the respective molarity ranges givenabove acidified by HCl to the given pH range. Preferred electrochemicalpotentials are E<0.8 V S H E (most superconductors in the solutionsdescribed will take up this potential naturally).

The time for which the superconductor is treated with the treatmentsolution will generally be in the range 1 to 10 minutes.

Most preferably the treatment is effected to produce an oxide film witha thickness in the range 1-10 μm.

We claim:
 1. A method of treating an oxide superconductor to preventdegradation thereof characterized in that it comprises dipping thesuperconductor in a solution of metal ions, selected from the groupconsisting of Mn²⁺, Ti³⁺, Pb²⁺ and Sn²⁺, having a pH of less than 6 andsimultaneously applying an external potential to form on those regionsof the surface area of the superconductor treated with the solution aninsulating, non-porous protective film of an oxide which is stableagainst further oxidation.
 2. A method of treating an oxidesuperconductor to prevent degradation thereof as claimed in claim 1characterized in that it comprises dipping the superconductor into asolution of metal ions which give rise to a protective oxide film andsimultaneously applying an oxidizing electrochemical potential.
 3. Amethod of treating an oxide superconductor to prevent degradationthereof as claimed in claim 1 characterised in that the metal ions areMn²⁺ in a solution of concentration 0.01 to 0.5M.
 4. A method oftreating an oxide superconductor to prevent degradation thereof asclaimed in claim 3 characterised in that the pH of the solution is <5.5. A method of treating an oxide superconductor to prevent degradationthereof as claimed in claim 3 characterised in that the treatmentsolution is equimolar Mn acetate and acetic acid.
 6. A method oftreating an oxide superconductor to prevent degradation thereof asclaimed in claim 1 characterised in that the metal ions are Pb²⁺ in asolution of concentration 0.01 to 0.05M.
 7. A method of treating anoxide superconductor to prevent degradation thereof as claimed in claim3 characterised in that the treatment solution is equimolar Pb acetateand acetic acid.
 8. A method of treating an oxide superconductor toprevent degradation thereof as claimed in claim 1 characterised in thatthe metal ions are Sn²⁺ in a solution of concentration 10⁻³ to 0.01M. 9.A method of treating an oxide superconductor to prevent degradationthereof as claimed in claim 8 characterised in that the pH of thesolution is <1.
 10. A method of treating an oxide superconductor toprevent degradation thereof as claimed in claim 9 characterised in thatthe treatment solution is SnCl₂ and hydrochloric acid.
 11. A method oftreating an oxide superconductor to prevent degradation thereof asclaimed in claim 1 characterised in that the metal ions are Ti³⁺ in asolution of concentration 10⁻³ to 0.1M.
 12. A method of treating anoxide superconductor to prevent degradation thereof as claimed in claim11 and the pH of the solution is <2.
 13. A method of treating an oxidesuperconductor to prevent degradation thereof as claimed in claim 9characterised in that the treatment solution is TiCl₃ and hydrochloricacid.
 14. A method of treating an oxide superconductor to preventdegradation thereof characterized in that it comprises dipping thesuperconductor in a solution of metal ions, selected from the groupconsisting of Mn²⁺, Ti³⁺, Pb²⁺ and Sn²⁺, having a pH of less than 6 andsimultaneously applying an external voltage to oxidize said metal.
 15. Amethod of treating an oxide superconductor to prevent degradationthereof as claimed in claim 14 characterised in that a potential of >0.8V (referred to the Standard Hydrogen Scale) is applied.
 16. A method oftreating an oxide superconductor to prevent degradation thereof asclaimed in any one of the preceding claims characterised in that thetime for which the superconductor is treated with the treatment solutionis in the range 1 to 10 minutes.